Although rodent populations are largely regulated by the emigration (or dispersal) of surplus individuals, the mechanisms of dispersal and the characteristics of the dispersants are not known. One leading hypothesis is the subordination-dispersal hypothesis, which predicts that young, subordinate individuals are forced from their birthplace into a dispersing, transient segment of the population, which suffers high mortality. Since evidence for this hypothesis is contradictory, the reproductive optimization hypothesis is proposed, which predicts that parents optimize their reproductive effort by dispersing when this will enhance their reproductive value. Dispersal will be measured in free-ranging populations of Peremyseus maniculatus bairdi by providing optimum artificial habitats that can be readily monitored for births and movements of the inhabitants. These habitats will consist of aluminum cylinders, two meters in diameters, that contain nest sites, ample food and protection from predators, but permit free entry and exit through a single gate. These cylinders will be arranged two meters apart in a hexagon. Six hexagons (mouse cities) will be established in an old field at distances approximately 50 meters apart. The age, sex and familial relationships will be ascertained for all individuals moving within and between hexagons. Only one half of the available habitats will be provided with a founding population. Under these optimal conditions, we predict that parents will disperse more frequently than their offspring. By altering the conditions in these habitats, the characteristics of the dispersants should change.